(1) Field of the Invention
The present invention relates to an image forming apparatus including a fixing device for fixing an unfixed image formed on a recording sheet onto the recording sheet by heat.
(2) Description of the Related Art
In image forming apparatuses such as printers or copiers adopting the electrophotographic method, typically a toner image corresponding to a piece of image data is transferred onto a recording sheet such as a recording paper sheet or an OHP sheet, and the unfixed toner image is then fixed onto the recording sheet by a fixing device. The fixing device heats the toner image on the recording sheet so that the toner image melts and is fixed on the recording sheet.
In recent years, a resistance heating method is adopted for a heater provided in the fixing device, wherein the resistance heating method uses a resistance heating body that emits heat when the current flows through it. Japanese Patent Application Publication No. 2009-109997 discloses a fixing device using heating belt (heat-emitting belt) that includes a resistance heating body. In this fixing device, an elastic roll is provided within the range of circulating movement of the heating belt that includes the resistance heating body. With this structure, the heating belt sandwiched by the elastic roll and a pressing roller makes a circulating movement. Also, a fixing nip, through which the recording sheet passes, is formed between the heating belt and the pressing roller.
An alternating current is supplied to both ends of the resistance heating body provided in the heating belt, wherein the two ends are positioned along the width direction perpendicular to the circulating movement direction of the heating belt (along the rotational axis direction of the heating belt, namely, along a direction perpendicular to the transportation direction of the recording sheet). The resistance heating layer emits Joule heat when an electric current is supplied thereto. The heat emitted by the resistance heating layer is applied to the recording sheet passing through the fixing nip. This allows the toner image on the recording sheet to be fixed thereon by the heat.
In this fixing device, the heating belt itself, on which the recording sheet is transported, emits heat, thus the distance from the resistance heating layer, which is the source of the heat, to the recording sheet is short. Accordingly, this structure enables the resistance heating layer to apply heat to the recording sheet efficiently, and the amount of energy consumption to be restricted during the warm-up and the fixing operation. The structure also shortens the warm-up time since the heating belt as the heat source has a small thermal capacity.
A fixing device using a resistance heating layer may have a damage such as a scratch in the resistance heating layer provided in the heating belt when, for example, a paper jam is inappropriately handled, or a foreign material is stuck on the recording sheet. If a long scratch occurs in the resistance heating layer and the scratch intersects with a direction in which the electric current flows in the resistance heating layer (with the width direction of the heating belt), the vicinities of both ends of the scratch have high temperatures locally.
The reason is as follows. When the resistance heating layer has a scratch extending along the circumferential direction, the current cannot cross the scratch to flow in the width direction of the heating belt, but flows bypassing the scratch. In that case, the electric current converges at the vicinities of both ends of the scratch, causing overheat at the both ends, and a local high-temperature state is produced.
When the heating belt is in such a local high-temperature state, image noise such as a high-temperature offset may occur. Also, when the scratch is further long, the current density at the vicinities of both ends of the scratch further increases, and an abnormal high-temperature state may be produced. In that case, the fixing device may be seriously damaged. For example, the surface of the pressing roller pressed against the heating belt may melt. For this reason, if a damage such as a scratch occurs in the resistance heating layer of the heating belt, it is preferable that the damage is detected soon to prevent occurrence of image noise, a damage of the pressing roller and the like.
When a scratch occurs in the resistance heating layer of the heating belt, the temperature in the vicinities of both ends of the scratch increases to create a local high-temperature region. As a result, it is possible to judge whether or not a scratch or the like has occurred in the resistance heating layer by detecting whether or not the heating belt has a local high-temperature region.
Japanese Patent Application Publication No. 2000-227732 discloses a structure where an infrared sensor is used to detect a surface temperature of a heating rotating body such as a heating belt. According to the disclosure of this document, the infrared sensor is disposed to face the surface of the heating rotating body in the state where the infrared sensor can move along the axial direction, and detects the surface temperature of the measurement region facing the surface of the heating rotating body in the rotating state.
By using the infrared sensor described in Japanese Patent Application Publication No. 2000-227732, it is possible to detect a temperature (average temperature or the like) while the heating belt rotates once (in one rotational period) in the measurement region, which is a partial region along the width direction of the heating belt, during the image formation (fixing operation). Accordingly, if it is possible to detect whether or not a temperature measured at the measurement region in the heating belt in one rotational period is higher than a predetermined threshold temperature, it is possible to detect whether or not a scratch has occurred in a part of the resistance heating layer corresponding to the measurement region. Here, if the measurement region of the infrared sensor is set over the whole region of the heating belt in the width direction, it would become possible to make the judgment on the occurrence of a scratch over the whole region of the resistance heating layer.
However, when the measurement region is set over the whole region in the width direction, an erroneous judgment is likely to occur. That is to say, when the recording sheet passes through the fixing nip, heat is removed from the heating belt by the recording sheet in the paper-passing region, but is not removed in the non-paper-passing region. Thus the surface temperature of the heating belt is higher in the non-paper-passing region than in the paper-passing region. Accordingly, when the recording sheet passes through the fixing nip, the temperature is measured to be higher in the non-paper-passing region than in the paper-passing region, and it may be erroneously judged that a scratch has occurred in the non-paper-passing region.
One conceivable measure for preventing the problem would be to set different thresholds for the paper-passing region and the non-paper-passing region. However, even if such a measure is taken, the temperatures of the heating belt measured in each rotational period may vary due to uneven thickness of the resistance heating layer in the circumferential direction, and it may be erroneously judged that a scratch has occurred in the resistance heating layer.
Also, when the fixing operation is continuously executed for a plurality of recording sheets, the plurality of recording sheets pass through the fixing nip with a predetermined gap therebetween. In that case, heat is not removed from the heating belt in the paper-passing region during a period of the predetermined gap between two recording sheets passing through the fixing nip. If such a state occurs while the temperature of the heating belt is measured in one rotational period, the temperature measured by the infrared sensor becomes high.
In that case, the temperature measured in the paper-passing region may exceed a set threshold temperature, and it may be erroneously judged that a scratch has occurred in the paper-passing region although in the actuality no scratch has occurred in the paper-passing region.
The present invention has been conceived in light of the above problems, and it is an object thereof to provide an image forming apparatus that accurately and unerringly judges whether or not an abnormality such as a scratch has occurred in the resistance heating layer.